1. Field of the Invention
The present invention relates to a method and apparatus for handling a scheduling information report, and more particularly, to a method and apparatus for handling resource allocation priority of the scheduling information report in a user equipment (UE) of a wireless communication system.
2. Description of the Prior Art
Long Term Evolution wireless communication system (LTE system), an advanced high-speed wireless communication system established upon the 3G mobile telecommunication system, supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, such as in base stations (Node Bs) alone rather than in Node Bs and RNC (Radio Network Controller) respectively, so that the system structure becomes simple.
In LTE system, a MAC Protocol Data Unit (PDU) consists of a MAC header, zero or more MAC Service Data Units (SDUs), zero or more MAC control elements, and optionally padding. A MAC PDU header consists of one or more MAC PDU sub-headers, each corresponding to either a MAC SDU, a MAC control element or padding. The MAC PDU sub-headers have the same order as the corresponding MAC SDUs, MAC control elements and padding. A MAC PDU sub-header corresponding to a MAC control element generally consists of two Reserved bits, an 1-bit Extension field and a 5-bit Logic Channel ID (LCID) field, i.e. 8 bits in total.
According to the current specification, MAC control elements transmitted by a UE include a buffer status report (BSR) MAC control element and a power headroom report (PHR) MAC control element. The BSR MAC control element is generated by a Buffer Status reporting procedure, and is used to provide the serving base station, or called enhanced Node B (eNB), with information about the amount of data in the uplink (UL) buffers of a UE for scheduling of uplink transmission. The PHR MAC control element is generated by a Power Headroom reporting procedure, and is used to provide the serving eNB with information about the difference between the maximum UE transmission (TX) power and an estimated TX power for Uplink Share Channel (UL-SCH).
There are three types of BSR for different triggering events, a regular BSR, a periodic BSR and a padding BSR. The regular BSR is triggered when UL data arrives at the UE transmission buffer and the UL data belongs to a logical channel with higher priority than those for which data already existed in the UE transmission buffer, or is triggered when a serving cell change occurs. The periodic BSR is triggered when a PERIODIC BSR TIMER expires. The padding BSR is triggered when UL resources are allocated and an amount of padding bits is equal to or greater than the size of the BSR MAC control element.
According to different requirements, the BSR MAC control elements can be categorized into two formats: short and long. A short BSR MAC control element is 1-byte long, and has 8 bits, where the former 2 bits indicate one Logic Channel Group which buffer status is being reported, and the remaining six bits indicate the amount of buffered data available across the Logic Channel Group. A long BSR MAC control element is 3-byte long, and utilized for reporting data amount of the UL buffers of all the Logic Channel Groups.
For the regular and periodic BSR, the format of the BSR MAC control element is determined by the UE according to whether there is more than one Logic Channel Group with uplink buffered data in the UE when the BSR is transmitted. If there is only one Logic Channel Group with uplink buffered data, the short BSR MAC control element is reported. Conversely, if there are more than one Logic Channel Groups with uplink buffered data, the long BSR MAC control element is reported.
For the padding BSR, the BSR MAC control element format is determined according to the number of padding bits remaining in the MAC PDU. If the number of padding bits is equal to or larger than the size of the short BSR MAC control element but smaller than the size of the long BSR MAC control element, the short BSR MAC control element is used, for reporting data amount in the uplink buffer of a highest priority Logic Channel Group. Conversely, if the number of padding bits is equal to or larger than the size of the long BSR MAC control element, the long BSR MAC control element is used, for reporting data amount in the uplink buffer of all the Logic Channel Groups.
After the BSR is triggered, if the UE has UL resources allocated for a new transmission for a current Transmission Time Interval (TTI), the UE generates a BSR MAC control element to report information about the amount of data in the UL buffers, such that the network can determine the total amount of data available across one or all logical channel groups. Conversely, if the UE does not have UL resources allocated for a new transmission for this TTI and the triggered BSR is a Regular BSR, a Scheduling Request (SR) procedure shall be triggered, for requesting the network to allocate an uplink grant for the UE. As a result, the BSR can be transmitted.
On the other hand, a PHR is triggered if any of the following events occur: (1) a timer “PROHIBIT_PHR_TIMER” for prohibiting power headroom reporting expires or has expired and a path loss of the UE has changed more than a predetermined value “DL_PathlossChange” since the last power headroom report; and (2) a PERIODIC PHR TIMER expires, in which case the PHR is referred below to as “Periodic PHR”. After the PHR is triggered, if the UE has UL resources allocated for anew transmission for this TTI, the UE obtains the value of the power headroom from the physical layer to generate a PHR MAC control element, and restarts the timer “PROHIBIT_PHR_TIMER”. Besides, if the triggered PHR is a “Periodic PHR”, the UE restarts the PERIODIC PHR TIMER. It should be noted that when periodic Power Headroom Reporting is configured, the first report should be included immediately when the UE has a grant for a new transmission.
When both BSR and PHR are triggered, since the PHR MAC control element requires 2 bytes for transmission (i.e. one byte PHR MAC control element with one byte MAC sub-header), an uplink grant of at least 4 bytes (a short BSR MAC control element plus its MAC sub-header and a PHR MAC control element plus its MAC sub-header) or 6 bytes (a long BSR MAC control element plus its MAC sub-header and a PHR MAC control element plus its MAC sub-header) are needed by the UE.
In some cases, the UL grant may be insufficient to include both two reports:                (1) The UL grant is requested by sending Scheduling Request (SR) via Physical Uplink Control Channel (PUCCH). Because eNB doesn't know which MAC control elements, e.g. short BSR, long BSR, and/or PHR, are triggered in the UE side, it may allocate the UL grant with the size smaller than 6 bytes to prevent resource waste.        (2) The UL grant is requested by sending SR via a Random Access (RA) procedure. When the Random Access procedure is initiated due to uplink data arrival, the UE first transmits a RA preamble to request an uplink grant and transmits the uplink data by a message 3 (Msg3) on Uplink Share Channel (UL-SCH). Since the smallest size of the UL grant for Msg3 is 80 bits, after including a Cell Radio Network Temporary Identifier (C-RNTI) and a Cyclic Redundancy Check (CRC) code, only 4 bytes are left. Consequently, a long BSR MAC control element and a PHR MAC control element can't be included in the same Transport Block (TB), i.e. MAC PDU, for transmission.        
According to the current specification, a priority between BSR and PHR is not specified. Besides, when a Regular BSR and a PHR are triggered, if the UL grant is 4 bytes left and the priority of the PHR is higher than that of the Regular BSR, in addition to a PHR MAC control element, a short BSR MAC control element triggered by a padding BSR will also be transmitted. Then, the BSR trigger is no longer pending. At this time, if the UE has more than one Logic Channel Groups with uplink buffered data, since the short BSR MAC control element can only provide information about buffered data amount of one Logic Channel Group, the network has to wait for the next BSR MAC control element sent by the UE to know complete data amount in the uplink buffer of the UE. It results in bad resource scheduling and bad UL transmission performance of the UE.